There is a tonometer for measuring intraocular pressure of an examinee's eye by blowing a fluid at a cornea of the examinee's eye through a nozzle and then optically detecting a deformed state of the cornea. As such tonometer, a device having an optical system for measuring corneal thickness by imaging (picking up) a cross-sectional image of an anterior segment of an examinee's eye including a cornea has been proposed. This device is arranged to correct measured intraocular pressure based on the measured corneal thickness (see WO 95/20342 (JP National publication No. 8(1996)-507463) and others).